chemical questions

So I just invested in a darkroom and am excited to get going with it. It came with a ton of stuff, icluding some chemicals. I have darkroom exprerience, just not my own stuff.
What is the differnece between D19 and D76. I have both. I know it say that the D19 is high contrast, but is that the only difference? What do yuopu guys use? I also got some Rodinal and Neutol Plus in concentrated bottles.

I also got half bottles of Kodafix and Indicator stop bath. How long does this stay good for in concentrated form? I got some new fixer poweder just in case it isnt good.

The shop I also go to has Illford chemicals. Do you guys recommend these over the kodak?